MARINE PROCESS VARIABILITY IN AN ENIGMATIC, SHELF-ISOLATED SANDSTONE: THE LATE TURONIAN WALL CREEK-TURNER SYSTEM, WY

Relatively thin, laterally extensive sandstones enveloped by marine shales are common features in Late Cretaceous strata of the U.S. Western Interior, and the sedimentary processes and associated depositional environments responsible for their formation has been the subject of much debate. One such unit is the Upper Turonian Wall Creek-Turner system in eastern Wyoming, consisting of the Wall Creek Member of the Frontier Formation and the coeval Turner Sandy Member of the Carlile Shale. Paleogeographic reconstructions of this system vary because regional studies of the Wall Creek-Turner system are sparse, and most analyses of the units derive regional interpretations of depositional environment based on localized outcrop belts or cored sections from a single hydrocarbon field. As such, the nature of regional stratigraphic relationships, facies characteristics, and depositional environments represented in the system remain unresolved.

This study incorporates sedimentological and ichnological descriptions of 19 outcrop and 41 cored sections of the Wall Creek-Turner system across the 250 km-wide Powder River Basin to characterize and quantify the depositional environments documented by the unit. Eleven environmental facies associations with 29 subordinate facies and distinct ichnological trends were identified in outcrop and core, documenting a range of shallow marine and offshore processes with varying degrees of deltaic influence. To the west, the unit was deposited by a mixed-process deltaic system displaying evidence of rapid sedimentation rates under predominately wave and tidal influence with significant modification by storm waves. To the south, evidence of deltaic influence wanes alongside an increase in deposition and modification by offshore processes including storm waves and sediment gravity flows under moderate to rapid sedimentation rates. To the east, the unit displays increasingly non-deltaic marine processes under variable sedimentation rates and modification by storm waves and bottom currents. This study illustrates the value of establishing regional facies frameworks when studying basin-isolated sandstones, and can be applied to many shelf-isolated sandstones of the U.S. Western Interior and beyond to generate more comprehensive depositional models and associated paleogeographic reconstructions.